Halter your horse. Turn on the thermometer by depressing the "on" button. Ideally, with a helper at the head of your horse, make contact with your horse's left shoulder and slowly move down the left (near) side of horse.

Lubricate the thermometer with lubricant or saliva. With your left hand, grasp the tail and slowly lift it. I use my elbow propped on the horse's hip as a contact point. I insert the thermometer into the anus with a gentle twisting motion of my right hand, about 1/2 of the length in.

Keep soft pressure with your finger on the thermometer so it doesn’t slip out, or gently push it back in if it is pushed out. Stand with the tail partially raised and the other hand stabilizing the thermometer until it beeps. Remove and read.

Digital thermometers are common and inexpensive and provide a quick reading. However, given their shape, you must hold them in the horse's anus as you take the temperature. The shape also makes it difficult to insert at an appropriate depth. This is why I still prefer mercury thermometers. That said, mercury thermometers are being phased out, so it is good to learn to use a digital thermometer.

The thermometer should be lubricated with spit or lubricant. A twisting motion often makes it easier to insert, but do not force it. For difficult horses, you must first desensitize the area. If a horse will not allow you to raise and handle its tail, it will not allow you to insert the thermometer. It is best to teach horses to allow you to take their temperature when they are healthy.

Erroneously low temperatures should prompt you to retake of the temperature. If the thermometer tip is against a fecal ball or not inserted far enough, the temperature may read artificially low.